The Labour MP Tom Watson, who believed he was on a News International 'enemies list' at the time. Photograph: Carl Court/AFP/Getty Images

The News of the World journalist Mazher Mahmood commissioned surveillance on his paper's chief phone-hacking critic, the Labour MP Tom Watson, in the hope of finding him having an affair, according to email evidence Watson has obtained.

News International's internal investigating group, the management and standards committee, belatedly turned over the emails to a parliamentary committee of which Watson was a member. They implicate Mahmood and two former NoW executives, the assistant editor Ian Edmondson and news editor James Mellor.

This latest revelation of methods at the now-closed NoW will present difficulties for John Witherow, the editor of the Sunday Times. Mahmood, the so-called "fake sheikh" who specialised in controversial undercover investigations, was rehired by the Sunday Times after its sister paper was closed down by Rupert Murdoch, and is still working there.

Witherow has not so far commented on the disclosures.

The attempt by journalists at the NoW to gain evidence of sexual indiscretions by its arch-critic was launched on the morning of Saturday 26 September 2009, at the start of the Labour party conference. Mahmood claimed in an email to Mellor, copied to Edmondson, that he had received a tip that married Watson was "shagging" a fellow activist, and that he was "creeping into her hotel" at Brighton. The information, from a so-far unknown purported informant, appears to have been completely false.

Mahmood described the MP as a "close lackey" of the then prime minister, Gordon Brown, and noted he was "anti-Blair". It was agreed that a private detective, the former police officer Derek Webb, known as "Silent Shadow", would be hired to stalk Watson through the conference, from 28 September to 2 October, in what proved to be a vain hope of getting confirmation. Had the story been substantiated and published, it would have destroyed his reputation.

According to the emails in Watson's possession, Edmondson described the prospect as a "great story" and added: "You might want to check his recent cutts [cuttings], v interesting!"

Watson at the time believed he was on News International's "enemies list". He was pursuing a libel suit against the Sun for falsely accusing him of involvement in organising online smears against the Conservatives. He was also vigorously pursuing News International on the culture, media and sport committee, where a series of Murdoch executives were mounting an ultimately unsuccessful cover-up of phone-hacking.

Peter Mandelson told the Leveson inquiry on Monday how Rebekah Brooks would "come on to me and complain" that Watson and his colleagues were "hounding" them, and demand: "Couldn't they be pulled away, pulled off?" Brooks, editor of the Sun at the time, had taken over as chief executive of NI at the beginning of September, in control of both Murdoch tabloids.

On the evening of 29 September, while Webb was still shadowing Watson at his conference hotel, the Sun revealed it was switching political sides, and published a dramatic front page headlined "Labour's lost it". From then on, the paper embarked on a ferocious campaign against Brown and his supporters.

He was sent down to Brighton on the Thursday of the conference to tail Watson but failed to locate him at his hotel.

"Then they got me to go around the hotels to see if I could find this woman … I went to one hotel where a bloke said she was staying and was told that everything was full up and that they had recommended a place further out from Brighton in Worthing.

"When I found the place I emailed the office and they booked me in. I never saw a thing for two or three days."

He says he was not commissioned by Mahmood, but somebody on the newsdesk.

Watson told the Leveson inquiry in his witness statement: "Recent disclosure from the company shows that the covert surveillance was commissioned by Mazher Mahmood with someone called Conrad acting as an accomplice. Mahmood made untrue and damaging claims in internal emails with the subject 'Labour sex scandal'. He claimed I was having an affair with a devout Muslim woman."

Mahmood previously testified to Leveson that Conrad Brown, a freelance video technician, was hired by the News of the World to provide covert video equipment and assist with surveillance.

James Murdoch has apologised at Leveson for the surveillance of Watson.

Watson also described the experience of former Labour MP Martin Salter after Salter refused to back Brooks's "Sarah's law" anti-paedophile campaign in the News of the World. Private investigators went to "extraordinary lengths" to find information on him, he said. Watson cited a report in the Guardian, disclosing that a private detective, Glen Lawson, had sought police records of any past convictions of Salter, as well as of Gordon Brown and former Labour minister Nick Brown.

News International's lawyer Rhodri Davies QC told the inquiry that the evidence merely showed the journalists acting on a tip for a story. He said NI was not pursuing Watson as a result of his membership of the culture, media and sport committee, and there was no evidence to support other allegations that all the committee members were being put under surveillance.